I've rarely had a season change that destroyed all my 'growing crops' in 40d, that's occasionally wiped out my seed stock for a type of crop.
You've got two answers, one's more work and the other's more waiting. The fast answer is to gather plants aboveground, if you have plants up there, and hope that you have the right sort of terrain to grow 'marsh type' plants. If your fortress is in a swamp, you're set. If not, concentrate your gathering within a tile or two of the edge of ponds or your brook/stream/river. If you're in badlands, mountains, or a glacier, this answer won't work at all for you. You're looking for rope reed, and it's fairly rare. Good news is that if you can harvest any rat weed or fisher berry plants, then your environment will eventually give you rope reeds, just keep gathering near the wetter areas.
Once you've got a rope reed plant, make a farm aboveground and process plants to get the seed or two for planting.
The easier and slower choice is to simply wait for the caravans to come. Human and elven caravans may carry rope reed seeds, and there's a chance these will grow in your outside soil even if you're not in a marsh. And your dwarven caravan can carry more pigtail seed, and definately will if you order it.